Use of a motion phantom to verify dose accuracy in different delivery techniques for lung tumors in stereotactic body radiation therapy

Background: Stereotactic body radiation therapy (SBRT) is the standard treatment for inoperable early-stage non-small cell lung cancer (NSCLC). However, respiration-induced tumor motion results in a nonuniform cumulative dose delivery to a lung tumor. In this study, we used a motion phantom system to investigate the effect of respiration-induced lung tumor displacement on dose delivery in the following three different delivery techniques in SBRT: Intensity-modulated radiation therapy (IMRT) SBRT (IMRT-SBRT), volumetric modulated arc therapy (VMAT) SBRT (VMAT-SBRT), and helical tomotherapy (HT) SBRT (HT-SBRT). Methods: We developed a tumor phantom and used a commercial motion phantom system for dose calculations and measurements at three different respiratory amplitudes (10, 14, and 18 mm). Four-dimensional computed tomography (4DCT) was performed to define tumor displacement during breathing for internal target volume (ITV) delineation. IMRT-SBRT, VMAT-SBRT, and HT-SBRT plans were generated for the target with different respiratory amplitudes. A total dose of 60 Gy in 5 fractions was prescribed to the target. An ionization chamber and radiochromic films were used to measure the point dose at the tumor center and the dose profiles, respectively, which were then compared with the calculated doses to verify the accuracy of the dose delivered to the lung tumor. Results: The HT-SBRT plans showed better conformity index (CI) and homogeneity index (HI) than the IMRT-SBRT and VMAT-SBRT plans for all respiratory amplitudes as follows: The mean CI values were 1.22±0.10, 0.95±0.02, and 0.95±0.02, and the mean HI values were 1.14±0.01, 1.20±0.01, and 1.20±0.01 for the HT-SBRT, IMRT-SBRT, and VMAT-SBRT plans, respectively. The results showed that in all three techniques at different respiratory amplitudes, the point dose differences at the tumor center between the calculated and measured doses were all within ±3% as follows: IMRT-SBRT =1.7%±0.7%, VMAT-SBRT =1.1%±1.4%, and HT-SBRT =-0.7%±0.7%. Moreover, regarding the dose profile measurements at different respiratory amplitudes, all film results showed that an adequate treatment dose coverage was obtained in the lung tumor (100% isodose curve encompassed the lung tumor). Conclusions: All SBRT plans generated based on 4DCT images ensured adequate treatment coverage, and the point dose differences between the calculated and measured doses were all within ±3% despite respiratory motion during beam delivery. The HT-SBRT plans showed better HI, CI, and agreement between the measured and calculated doses than the IMRT-SBRT and VMAT-SBRT plans at different respiratory amplitudes.